1. Field of the Invention
This invention relates to a process for the production of light-stable, solvent-resistant polyurethane-urea elastomer coatings by reactive coating wherein NCO prepolymers are reacted with diamino-C.sub.1 -C.sub.4 -tetraalkyldicyclohexylalkanes containing up to 4 carbon atoms in the alkyl group with continuous mixing and continuous discharge of the reaction mixture.
2. Description of the Prior Art
The coating of substrates such as leather, skiver, textile materials or nonwovens with polyurethanes by reactive coating has been known for some time, cf. for example DE-PS No. 872,268 (Example 8), DE-PS No. 957,564, DE-A No. 1,023,449, DE-PS No. 1,240,656, US-PS No. 3,194,793 and, in particular, DE-AS No. 1,570,524. In the process described in DE-AS No. 1,570,524, an NCO prepolymer optionally dissolved in a solvent is sprayed together with an organic diamine dissolved in a solvent onto the substrate to be coated, the equivalent ratio of amino to hydroxyl groups being above 0.5 and the equivalent ratio of isocyanate groups to the sum of hydroxyl and amino groups being between 0.95 and 1.6.
On a laboratory scale polyurethane urea elastomers may be produced from a variety of the starting components described in the publications cited above mixed in various ratios with one another. Unfortunately, formulations such as these generally cannot be applied with any success at an industrial scale because, on the one hand, particular attention has to be paid to the processibility of the reactants (for example their viscosity, especially their reactivity) and, on the other hand, the stringent physical property requirements of the polyurethane coatings. The difficulty in this regard is that the desired properties of the polyurethane urea (for example hydrolysis stability, high flexural strength, tensile strength, low-temperature flexibility and light stability) conflict with one another i.e., products showing high hydrolysis stability and tensile strength generally show poor flexural strength and low-temperature flexibility and vice-versa.
In addition, reactivity problems often prevent the process from being carried out on an industrial scale. If the individual components are combined by external mixing, e.g. by whirling two streams of liquids together in air as is preferred according to DE-AS No. 1,570,524, the reactants generally cannot be thoroughly and uniformly mixed (P. Arbaud, Technicuir, No. 2, February 1970, page 29). Accordingly, zones having different physical properties, particularly with regard to light stability, flexural strength and tear strength, are formed in the coating (cf. H. Traubel, "Polyurethanes in Leather Finishing and Coating", J. of the Soc. of Leather Techn. and Chem. 57, page 61 (1973)).
According to DE-AS No. 26 37 115, the reactivity problems may be overcome by processing NCO prepolymers with blocked aliphatic amines or adducts containing amino groups. The aliphatic amines have to be blocked because the free aliphatic amines cannot be reacted with the NCO prepolymers in such a way as to form a film or a coating composition which levels uniformly and adheres sufficiently to the substrate. In addition, the reaction with the free aliphatic amines takes place so quickly that the mixing units immediately become clogged up with reaction products.
From DE-OS No. 29 45 614, page 11, it is known that cycloaliphatic compounds based on 4,4'-diamino-3,3',5,5'-tetraalkyldicyclohexylmethane (C.sub.1 -C.sub.4 alkyl groups), which are obtained by a process described in this publication, may be used for the production of polycondensation plastics such as polyamines, polyurethanes, polyureas and epoxy resins.
DE-OS No. 3,011,711 describes an improved process for coating substrates with polyurethane ureas by reacting an NCO prepolymer based on a mixture of a diisocyanate and 2 to 20 mole % of a triisocyanate with the partly blocked hardener system described in DE-OS No. 2,637,115. The triisocyanate used is a trimer of tolylene diisocyanate or a mixed trimer of tolylene diisocyanate and hexane diisocyanate. Preferred NCO prepolymers are based on tolylene diisocyanate.
DE-OS No. 33 09 992 describes a process for coating skiver which comprises applying a top coat to a temporary support and then a reactive mixture which only forms a high molecular weight polyurethane from the top coat and then pressing the skiver into the reacting mass. The reactive system contains blocked NCO prepolymers and cycloaliphatic diamines such as dicyclohexyl methane diamines. The blocked NCO prepolymers are synthesized using aromatic polyisocyanates and are preferably blocked by ketoximes (cf. for example DE-OS Nos. 28 14 079, 29 02 090, 31 34 161 and 30 04 327).
However, the reaction of NCO prepolymers based on aromatic diisocyanates with aliphatic or cycloaliphatic diamines is so fast that either the prepolymers have to be blocked (for example with ketoximes) or the diamines have to be blocked (as completely or partly blocked ketimine derivatives). However, the use of blocked components involves numerous disadvantages. With ketoxime-blocked NCO prepolymers, the ketoxime is split off during the reaction, while with ketimine-blocked diamines, the starting ketone has to be split off in a hydrolysis reaction which is not always easy to control. The risk of environmental pollution by the decomposition products necessitates additional technology for retaining the decomposition products.
Accordingly, an object of the present invention is to provide a reactive coating system which does not have the disadvantages of blocked systems, is highly light-stable and does not discolor and is resistant to solvents, but which may be safely processed without interruptions in conventional metering and mixing-and-spraying units.
It has now surprisingly been found that unblocked isocyanate prepolymers based on aliphatic or cycloaliphatic diisocyanates and, optionally, relatively low percentages of polyisocyanates, which contain only NCO groups of (cyclo)aliphatic reactivity in the NCO prepolymer may be reacted in the form of highly concentrated solutions to form polyurethane urea coatings by continuously mixing the reactants in suitable mixing units and applying the resulting mixtures to a temporary support or directly onto the substrate using the reactive coating technique provided that diamino-C.sub.1 -C.sub.4 -tetraalkyldicyclohexylalkanes are used as chain extending agents. The NCO prepolymers should have a negligible content of monomeric, aliphatic or cycloaliphatic diisocyanates, i.e. free monomeric diisocyanate contents of &lt;2.0% by weight, preferably &lt;0.5% by weight.